The present invention can be applied to a broad range of semiconductor devices, although it is especially effective in light-receiving elements such as photodiodes and solar cells. The background of the invention is described below with reference to solar cells as a specific example of the prior art.
A conventional solar cell structure with a p-type base has a negative electrode that is typically on the front-side or sun side of the cell and a positive electrode on the back side. It is well known that radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate hole-electron pairs in that body. Because of the potential difference which exists at a p-n junction, holes and electrons move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metallized, i.e., provided with metal contacts that are electrically conductive.
Most electric power-generating solar cells currently used on earth are silicon solar cells. Process flow in mass production is generally aimed at achieving maximum simplification and minimizing manufacturing costs. Electrodes in particular are made by using a method such as screen printing from a metal paste.
An example of this method of production is described below in conjunction with FIG. 1.
FIG. 1 shows a p-type silicon substrate, 10.
In FIG. 1(b), an n-type diffusion layer, 20, of the reverse conductivity type is formed by the thermal diffusion of phosphorus (P) or the like. Phosphorus oxychloride (POCl3) is commonly used as the phosphorus diffusion source. In the absence of any particular modification, the diffusion layer, 20, is formed over the entire surface of the silicon substrate, 10. This diffusion layer has a sheet resistivity on the order of several tens of ohms per square (Ω/□), and a thickness of about 0.3 to 0.5 μm.
After protecting the front surface of this diffusion layer with a resist or the like, as shown in FIG. 1(c), the diffusion layer, 20, is removed from the rest of the surfaces by etching so that it remains only on the front surface. The resist is then removed using an organic solvent or the like.
Next, a silicon nitride film, 30, is formed as an anti-reflection coating on the n-type diffusion layer, 20, to a thickness of about 700 to 900 Å in the manner shown in FIG. 1(d) by a process, such as plasma chemical vapor deposition (CVD).
As shown in FIG. 1(e), a silver paste, 500, for the front electrode is screen printed then dried over the silicon nitride film, 30. In addition, a back side silver or silver/aluminum paste, 70, and an aluminum paste, 60, are then screen printed and successively dried on the back side of the substrate. Firing is then typically carried out in an infrared furnace at a temperature range of approximately 700 to 950° C. for a period of from several minutes to several tens of minutes.
Consequently, as shown in FIG. 1(f), aluminum diffuses from the aluminum paste into the silicon substrate, 10, as a dopant during firing, forming a p+ layer, 40, containing a high concentration of aluminum dopant. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell.
The aluminum paste is transformed by firing from a dried state, 60, to an aluminum back electrode, 61. The back side silver or silver/aluminum paste, 70, is fired at the same time, becoming a silver or silver/aluminum back electrode, 71. During firing, the boundary between the back side aluminum and the back side silver or silver/aluminum assumes an alloy state, and is connected electrically as well. The aluminum electrode accounts for most areas of the back electrode, owing in part to the need to form a p+ layer, 40. Because soldering to an aluminum electrode is impossible, a silver or silver/aluminum back electrode is formed over portions of the back side (often as 5-6 mm wide busbars) as an electrode for interconnecting solar cells by means of copper ribbon or the like. In addition, the front electrode-forming silver paste, 500, sinters and penetrates through the silicon nitride film, 30, during firing, and is thereby able to electrically contact the n-type layer, 20. This type of process is generally called “firing through.” This fired through state is apparent in layer 501 of FIG. 1(f).
As noted above, the back surface electrode that is used to interconnect the solar cells through soldering may comprise Ag or Ag/Al compositions. When prior art Ag compositions are used, they can provide good solderability and adhesion. However, since the Ag compositions cannot produce a back surface field, conversion efficiency of the solar cell suffers. On the other hand, when Ag/Al compositions are used, adhesion strength is generally lowered and gives rise to concerns regarding long term reliability. This is due to the fact that the addition of Al will generally hurt solderability and thus, adhesion performance.
Furthermore, there is an on-going effort to provide compositions which are Pb-free while at the same time maintaining electrical performance and other relevant properties of the device. The present inventors provide novel Ag/Al comprising composition(s) and semiconductor devices which simultaneously provide such a Pb-free system while still maintaining electrical performance, and improving adhesion. The current invention provides such compositions and devices. Furthermore, the composition(s) of the present invention lead to reduced bowing in some embodiments of the invention.